A. Field of the Invention
The present invention relates generally to a circuit for measuring frequency variations in a high frequency signal and, more particularly, to a circuit for measuring the response from a quartz crystal microbalance gas sensor.
B. Description of the Related Art
Piezoelectric acoustic wave devices, including quartz crystal microbalance (QCM) devices, have been used to measure the concentration of moisture and particulates in gas streams, the accumulation of solids in fluid compositions, and similar applications for measuring the concentration of a substance. These acoustic wave devices typically are coated with a material that has an affinity with the substance whose concentration is being measured. The acoustic wave device operates by measuring the amount of the substance deposited onto the surface of the device. This deposit increases the mass of the acoustic wave device and thereby changes the frequency at which the piezoelectric acoustic wave device will resonate. When the acoustic wave device is incorporated into an electrical oscillator circuit, this change in resonant frequency causes the frequency at which the oscillator is operating to change. Thus, the concentration of the substance being measured can be determined by measuring the change in operating frequency of the piezoelectric oscillator circuit.
These acoustic wave devices can be constructed to respond to very low concentrations of the substance being measured, producing a very sensitive sensor. However, when low concentrations are being measured, the change in operating frequency of the piezoelectric oscillator is correspondingly small. The difficulty of measuring these small variations in frequency is increased because of the relatively high frequency at which the piezoelectric oscillator operates, caused by the high resonant frequency of the acoustic wave device.
Several techniques have been used to measure this frequency variation. One method involves resonant frequency determination based upon impedance measurements. This technique is an analog-digital hybrid circuit that is prone to noise, is complex, and expensive to implement. The use of a frequency counter avoids these drawbacks by providing a low cost fully digital circuit that has high noise immunity. However, to accurately measure small variations in the high frequency signals produced by the piezoelectric oscillators, a frequency counter with greater than 16-bit capacity is required, increasing the cost of the circuit.
One method to overcome this problem is to provide two acoustic wave devices, and arrange the devices so that the substance being measured is only deposited onto one of the devices, the other device being a reference (the reference device may be constructed without a coating to attract the substance or with a sealing coating to prevent the substance from being deposited on its surface, or may be positioned where it will not be exposed to the substance being measured). The acoustic wave devices are each connected to a separate oscillator. A frequency measuring circuit receives the output signals from the two oscillators, mixes the two signals to produce a difference frequency signal (having a frequency equal to the difference in frequency between the two oscillator signals), and then measures the frequency of the difference frequency signal. Because the difference frequency is much lower than the oscillator frequencies, a 16-bit counter can be used while still providing the required resolution. However, this approach results in a larger dynamic range for the mixed signal and also a more complex and costly system.
The present invention overcomes these problems by providing a novel circuit and method for measuring small variations in a high frequency signal. Accordingly, it is an object of the present invention is to provide an electrical circuit and method for accurately measuring small variations in frequency of high frequency signals. It is also an object of the invention to provide a low cost fully digital frequency measuring circuit that has a high noise immunity. Another object of the invention to provide a frequency measuring circuit that can be constructed by the simple integration of commercially available components.
A further object of the invention is to provide an apparatus and method for measuring low concentrations of a gas. Another object of the invention is to provide an apparatus for measuring gas concentration that is low cost, reliable, and constructed from commercially available components.